Why do I have the 2.80 API on blender 2.79
Blender version 2.79b is the one that you can download when you click Download Blender at the top of the download page at https://www.blender.org/download/ at the time I am writing this (05-01-2019). 

If you however choose to scroll down to the bottom of that page and click the red button 'Latest Experimental Builds' in the section with the red background and a clear disclaimer in English: 

 you will be downloading experimental builds (not unlike the disclaimer states) and may try latest changes that may include changes to the Python API.